The use and mechanism of reversible chain transfer agents for free radical polymerization is now generally known and coined as RAFT (Reversible Addition Fragmentation Transfer), see, for example, U.S. Pat. No. 6,153,705, WO 98/01478, WO 99/35177, WO 99/31144, and WO 98/58974, each of which is incorporated herein by reference. Recently new agents have been disclosed which are readily available for polymerizing desired monomers under commercially acceptable conditions, which include high conversion at the shortest possible reaction times and lower temperatures, see, for example U.S. Pat. Nos. 6,380,335, 6,395,850, and 6,518,364, each of which is incorporated herein by reference. Dithio compounds used in RAFT polymerizations can provide material with the attributes of living polymerization, i.e. low molecular weight distribution and block architectures.
There are currently some limitations to polymers produced by the RAFT process using dithiocarbonylated or dithiophosphorylated chain transfer agents. For instance, the polymers may contain one or more functional groups, such as dithioester, dithiocarbonate, dithiocarbazate, or dithiocarbamate, which absorb in the visible light. The appearance potentially limits the polymer's potential applications to such areas as electronics, which may require a transparent material. In some cases the dithio compound might be unstable under the conditions of use, or might degrade upon aging, and generate some unwanted effects such as odor, discoloration, etc.
WO 02/090397 discusses a method of radical reduction of dithiocarbonyl or dithiophosphoryl groups using a free radical initiator and a compound bearing a labile hydrogen atom. The method disclosed essentially removes the unwanted group from the polymer chain end and replaces it with a hydrogen atom. It does not disclose or teach any other methods for removing unwanted groups from the end of a polymer chain, or replacing those moieties with anything other than hydrogen. The use of a hydrogen labile compound may be undesirable in some circumstances, such as when those compounds are mercaptans, notoriously odoriferous, or alcohols, which may be non-solvents of the polymers.
In addition, copending U.S. patent application Ser. No. 10/407,405, entitled “Cleaving and Replacing Thio Control Agent Moieties from Polymers made by Living-Type Free Radical Polymerization” filed on Apr. 3, 2003, (attorney docket number 2000-089CIP3) the disclosure of which is herein incorporated by reference in its entirety, focuses on the replacement of the thiocarbonylthio or thiophosphorylthio groups with an alkyl group that is the result of the fragmentation of a free radical initiator, for example, from lauroyl peroxide or AIBN, or an addition-fragmentation chain transfer agent. In this case, a radical formed by the thermal fragmentation of the initiator adds to the chain transfer agent, which liberates the polymer radical via fragmentation. The polymer radical then combines with another alkyl radical present in the medium to yield the thio moiety free polymer. This is an efficient method to remove a thio moiety from polymers such as polymethacrylates, but its application to some polymers, such as polyacrylates is limited, since a comparatively higher energy radical is needed in order to induce the cleavage of the thio moiety-polymer bond.
Thus, what is needed is a method for removing certain groups from a chain end of a wide variety of polymers and replacing them with other groups.